1. Field of the Invention
The present invention relates to a reflecting polygon mirror to be mounted on the optical scanner used for electrophotographic equipment or the like, and more specifically, to a film formed on the reflecting surface of the polygon mirror. The present invention also relates to an optical scanner and electrophotographic equipment.
2. Description of the Related Art
Hitherto, in the optical scanner such as a laser beam printer or the like, writing or reading of image information is made by scanning on the surface of an image carrier optically with optically modulated luminous flux (laser luminous flux) via a rotating polygon mirror as is disclosed in Japanese Patent Publication No. 62-36210.
FIG. 8 is a schematic drawing showing an example of the construction of the principal portion of an optical scanner. In FIG. 8, a luminous flux emitted from the light source 1 such as a semiconductor laser or the like is collimated by a collimator lens 2, is condensed by a cylindrical lens 83 that has a reflective power only in the vertical scanning direction, and is linearly projected on the deflecting/reflecting surface 4a of the light deflector 4 constructed of a polygon mirror or the like. The collimator lens 2 and the cylindrical lens 83 constitute an image forming optical system. The luminous flux deflected and reflected on the deflecting/reflecting surface 4a is introduced through a scanning lens 87 comprising a lens 87a having a spherical surface and a negative reflective power and a lens 87b having different reflective powers between two directions orthogonal with each other and a toric surface onto the scanned surface 89 to form a spot. Then, the deflector 4 is rotated about the axis of rotation 82 by means of a motor 85 in the direction shown by an arrow 86 to scan the deflecting scanning surface on the scanned surface 89 optically in the direction shown by an arrow 90 (horizontal scanning direction).
Materials used in the polygon mirror are in many cases aluminum, plastic, glass. Then, the reflecting surface is coated with a vapor deposited film or an oxide film on anode for increasing the reflectance, eliminating the angle dependence, and preventing oxidation.
Since the reflecting surface of the polygon mirror that is used as a light deflector is coated with a reflection increasing film or the like by vacuum deposition, the difference of film thickness due to errors in manufacture or the uneven film thickness due to the method of manufacture results in variations in reflecting characteristics, and thus it requires an evaporation apparatus of high accuracy, an increase in the basic reflectance by employing a multilayer structure for alleviating the impact therefrom, or a special method of evaporating is necessary, which results in increase in cost or decrease in the possibility of film design.
In a technique to apply an amorphous fluorochemical resin film coating over the oxide film on anode, the oxide film on anode decreases the angle dependence of the reflectance and the amorphous fluorochemical resin film increases the durability for dew condensation. This anodic oxidation coating step requires a large apparatus for washing of the polygon mirror sufficiently, which presents a problem in terms of cost.
In order to solve the problem described above, a rotating wet film-forming method in which a single layer film can be formed at lower cost than two exampled above is proposed. However, when a film is formed on the polygon mirror according to the rotating wet film-forming method, the entire reflecting surface is immersed in the coating liquid, and thus the surfaces other than the reflecting surface are also coated with the coating liquid. As a consequent, there are tendencies where the coating liquid applied on the surfaces other than the reflecting surface gathers on the reflecting surface at the time of the rotating film-forming operation, and resulting in significant variations in film thickness or accidental occurrence of irregular film configurations. Therefore, improvement of productivity and decrease in cost of the rotating wet film-forming method has been problems to be solved.
Recently, the range of the angles of incident of the light being projected on the polygon mirror tends to be widened due to the increased requirement of miniaturization of the optical scanner. On the other hand, various optical films are formed on the reflecting surface of the polygon mirror for obtaining angle-independent uniformity of reflectance, and those optical films have an angle at which light of P-polarization is completely transmitted (Brewster's angle), in other words, when the range of the angles of incident is set to the range including Brewster's angle, an angle that coincides with the reflectance of the lower layer exists absolutely. In the polygon mirror that is desired to have a constant reflectance for the range of the angles of incident, presupposing that a film having a uniform thickness is formed, when the angle of incident is wider than Brewster's angle, the range of variations in reflectance increases, and thus angle-independent uniformity of reflectance can hardly be obtained. Therefore, in the related art, it is difficult to obtain a polygon mirror that can accommodate wide range of angles of incident and thus it is difficult to obtain a miniaturized optical scanner.